An oil hydraulic circuit that includes a regeneration boom circuit is shown in FIG. 5 as an example. Such a circuit is typically used as a boom-down circuit in a hydraulic excavator.
Referring to FIG. 5, when a lever of an electric joystick 1 for performing boom-down operation is operated, signals from the electric joystick 1 are input through a controller 3 into a solenoid 2a of a first boom directional control valve 2, which is of a solenoid-operated 3-position, 6-port type. As a result, the first boom directional control valve 2 shifts upward so that hydraulic oil fed from a pressurized oil source 4 flows through a line 5 into the rod-side 7 of a boom cylinder 6. The hydraulic oil in the head-side 8 of the boom cylinder 6 flows through lines 9, 10 into a tank line 11 to cause a rod 12 to contract.
When signals from the electric joystick 1 are input through the controller 3 into a solenoid 14 of a regeneration boom valve 13 simultaneously with the input of signals into the solenoid 2a of the first boom directional control valve 2, the regeneration boom valve 13 shifts upward. As a result of the shifting of the regeneration boom valve 13, during the period when the pressure in the rod-side 7 is lower than the pressure in the head-side 8, a part of the return oil from the head-side 8 passes through a passage 15 and a check valve 16, which is provided in the regeneration boom valve 13, and joins the hydraulic oil fed from the pressurized oil source 4. The combined hydraulic oil flows through the line 5 into the rod-side 7. Therefore, compared with oil hydraulic circuits that are not provided with such a regenerating circuit, a greater amount of oil is fed into the rod-side 7, enabling the boom to be lowered at a higher speed.
At that time, a considerable amount of oil, i.e. the regenerated oil that is not required to be fed into the rod-side 7, is returned through a return-oil control orifice 17 of the first boom directional control valve 2 to the tank line 11, because the cross-sectional area of the single-rod type boom cylinder 6 is greater at its head-side 8 than at its rod-side 7 by the cross-sectional area of the rod 12.
Operating an electric joystick 18, which is provided to perform boom-up operation, causes signals to be input through the controller 3 into the solenoid 2b of the first boom directional control valve 2 in the same manner as the boom-down operation described above. As the first boom directional control valve 2 shifts downward, the hydraulic oil fed from the pressurized oil source 4 flows through a line 9 into the head-side 8, and the hydraulic oil in the rod-side flows through the line 5 and the line 10 into the tank line 11, causing the rod 12 to extend.
At the same time, signals from the electric joystick 18 are input into a solenoid 19a of a second boom directional control valve 19 in the same manner as described above, so that the second boom directional control valve 19, which is of a solenoid-operated 2-position, 4-port type, shifts downward. As a result, hydraulic oil from a pressurized oil source 20 passes through a line 21 and a line 22 so as to join the hydraulic oil in the line 9 and flow into the head-side 8.
Numeral 23 denotes a directional control valve which is dedicated for another hydraulic actuator and connected in series or parallel with the second boom directional control valve 19. This directional control valve 23 may be a first arm directional control valve of a solenoid-operated 3-position, 6-port type and shares the hydraulic oil from the pressurized oil source 20 with the second boom directional control valve 19 when the arm is operated simultaneously with boom-up operation.
Numerals 24 and 25 denote directional control valves which are connected in parallel (connection in series is also possible) with the first boom directional control valve 2 and dedicated for other hydraulic actuators than the boom. These directional control valves may be of a solenoid-operated 3-position, 6-port type. In the case of the present example, they are a second arm directional control valve 24 and a bucket directional control valve 25 and share the hydraulic oil from the pressurized oil source 4 when their respective actuators are operated simultaneously with boom-up operation or boom-down operation.
The first arm directional control valve 23 and the second arm directional control valve 24 are adapted to shift their respective positions as a result of operating an arm-operating electric joystick (not shown) so that, in the same manner as the operation for raising the boom, the hydraulic oil from the directional control valves 23, 24 are joined and fed into an arm cylinder 26, while return oil flows to a tank. Thus, the rod of the arm cylinder 26 is contracted or extended.
The bucket directional control valve 25, too, is adapted to function in the same manner as above so as to cause a bucket cylinder 27 to contract or extend as a result of operating an electric joystick (not shown) for operating the bucket.
An oil hydraulic circuit having a configuration described above presents the following problem:    (1) when lowering the boom by the boom cylinder 6 while simultaneously operating another hydraulic actuator, such as a bucket, in the state that the bucket is off the ground, the oil fed from the pressurized oil source 4 is divided and distributed to the boom cylinder 6 and the bucket cylinder 27, causing such problems as a slower action of the bucket and reduced operation efficiency in comparison with operating the bucket alone;    (2) although a sufficient amount of regenerated oil can be fed to the rod-side 7 from the head-side 8 when the bucket is off the ground, the configuration described above, which calls for additionally feeding oil from the pressurized oil source 4, requires a considerable amount of excess oil to be returned to the tank line 11, resulting in wasted energy caused by unnecessary supply of fluid from a pump of the pressurized oil source 4;    (3) considerable skill is required to perform triple combined operation, which calls for simultaneously performing boom-up, arm-drawing, and bucket-opening and may be typically performed in a quarry to rake in gravel or debris; and    (4) when performing so-called slope tamping, i.e. hardening the ground by tamping it with the bottom of a bucket by lowering the boom, by repeatedly raising and lowering the boom, considerable skill is required to perform satisfactory slope tamping continuously, because failing to raise the boom at the precise moment that the bottom of the bucket touches the ground may result in various problems, such as tamping the ground too hard or causing the vehicle body to rise due to the reaction force resulting from the boom-down operation.
In order to solve the above problem, an object of the present invention is to prevent a decrease in efficiency of performance of a work machine, eliminate energy waste, and also facilitate operation that would otherwise require considerable experience or, skill.